Acid and Alkali

 

Acids and alkalis

1. Acids and alkalis are two important solutions.

 

2. Many people think that acids and alkalis are dangerous substances.

 

3. Actually, they are not only used in laboratories but also in many common items.

 

Acids

Alkalis

 In daily life

- Many foods contain acid. Acid in food gives a sour and tangy taste - tartaric acid in grapes; citric acid in oranges and lemons; lactic acid in yoghurt drinks

- Toothpastes, soaps and detergents, and antacid pills contain alkalis. Alkalis give these items a soapy and slippery feel.

 In
 laboratories

- Examples are hydrochloric acid, sulphuric acid and nitric acid

- Examples are sodium hydroxide, potassium hydroxide and calcium hydroxide

 Properties

- Has a sour taste

- Has a pH value less than 7

- Has a corrosive nature

- Turns blue litmus paper red

- Reacts with reactive metals such as magnesium and zinc to produce hydrogen gas

- Has a bitter taste

- Has a soapy and slippery feel

- Has a pH value greater than 7

- Has a corrosive nature

- Turns blue litmus paper red

- Reacts with ammonium salts to produce ammonia gas

 

The role of water in acids and alkalis

1. Acids and alkalis may exist in solids, liquids or gases.

 

2. However, acids only show their acidic properties when dissolved in water.

(a) For example, when a piece of dry blue litmus paper is added to solid citric acid, there is no change in colour.

(b) When water is added to the acid, the blue litmus paper turns red. This shows that acid only shows its properties in the presence of water.

 


3. Similarly, alkalis only show their alkaline properties when dissolved in water.

(a) When a piece of dry red litmus paper is added to solid barium hydroxide, there is no change of colour.

(b) When water is added, the red litmus paper turns blue. This shows that alkali only shows its properties in the presence of water.

 

 

Acidic and alkaline substances in daily life

1. The simplest way to determine whether a household substance is acidic or alkaline is by using either the pH paper or the universal indicator.

 

2. Dry substances must first be mixed with some distilled water before being tested.

 

3. An acidic substance has a pH value less than 7. The smaller the pH value, the more acidic is the substance.

 

4. An alkaline substance has a pH value greater than 7. The larger the pH value, the more alkaline is the substance.

 

5. If the pH value is exactly 7, the substance is neutral, that is neither acidic nor alkaline.


 

6. Figure and Table below show the uses of some acids and alkalis in our daily life.



                              Table 5.5 Uses of some common alkalis in daily life

Alkali

Uses of alkali

 Sodium hydroxide

- To produce soaps, detergents, plastic and rayon

 Calcium hydroxide (lime)

- To produce cement, mortar, soda lime and glass

- To neutralise acidic soil in agriculture

 Ammonia

- To prevent the coagulation of latex

- As a cleaning agent in glass and window cleaners

- To produce nitric acid and fertilisers

 Magnesium hydroxide

- To produce medicines such as antacids (to ease stomachache due to excess acid), milk of magnesia and Epsom salt

 Caustic soda (mixture of sodium hydroxide   and calcium hydroxide)

- As a cleansing agent in oven cleaners

         

Neutralisation

1. When an acid is added to an alkali, they cancel out each other's properties. This process is called neutralisation.

 

2. Neutralisation is a chemical reaction in which an acid and an alkali react form a salt and water. The general word equation for the neutralisation process is as follows:

                Acid + alkali → salt + water

 

3. In neutralisation, the acid loses its acidic properties and the alkali loses its alkaline properties.

 

4. Different acids and alkalis produce different types of salts. Here are some examples:

              Hydrochloric acid + sodium hydroxide → sodium chloride + water


             Sulphuric acid + potassium hydroxide → potassium sulphate + water


             Nitric acid + calcium hydroxide → calcium nitrate + water

 

5. To neutralise an acid or alkali completely, we use a method called titration. In this method, 

(a) the correct amounts of an acid and an alkali are mixed using a burette.

(b) an acid-alkali indicator is used to detect the end-point of neutralisation. Table below shows a few acid-alkali indicators that can be used.

               

Table 5.6   Common indicators and their colour changes


Indicator

 

Colours of indicators and their colour changes

Acid

Alkali

Neutral

 Litmus

Red

Blue

Purple

 Universal

Yellow

Purple

Green

 Methyl orange

Red

Yellow

Orange

 

Application of neutralisation

1. Red ant and bee stings are acidic. They can be neutralised by applying alkaline substances such as bicarbonate powder or calamine lotion.

 

2. Wasp stings are alkaline. They can be treated with acidic substances such as vinegar.

 

3. Hair is healthy and strong when it is slightly acidic but shampoos are usually slightly alkaline. Therefore, washing hair with alkaline shampoos makes hair look dull and coarse. Hair conditioners which are acidic can be used to neutralise the residue of shampoo on the hair, making hair look smooth, shiny and healthy.

 

4. Bacteria act on the residue of food in our mouth to produce acid that attacks the tooth enamel causing tooth decay. Therefore, toothpastes contain magnesium hydroxide that can neutralise the acid and prevent it from dissolving the tooth enamel.

 

5. Excessive acid in the stomach can cause indigestion and gastric pain. Antacids such as milk of magnesia contain alkalis that help to reduce the acidity in the stomach.

 

6. Most crops do not grow too well in acidic soil. Slaked lime (calcium hydroxide) is added to the soil to make it less acidic.